The present invention relates to a radio communication terminal or access terminal to be used in a radio communication system and a communication method for the radio communication terminal.
Radio communication systems for communicating are being rapidly introduced to various fields in recent years. Especially, CDMA (Code Division Multiple Access) radio communication systems, communicating information (audio information, etc.) by code division multiplexing the information by use of a plurality of spread codes, have recently become widespread, by which high-speed communication irrespective of time, place and the party has become possible. As a type of CDMA, 1×EV-DO (1×EVolution Data Only) radio communication system is known, for example. Since 1×EV-DO employs variable bit rate communication (best effort radio communication system), higher transmission rate is more beneficial to the users.
However, in best effort radio communication systems like 1×EV-DO, the transmission rate can not arbitrarily be designated and set by the user but varies depending on conditions of radio waves. Thus, techniques for easily checking the radio wave conditions and controlling the communication method depending on the radio wave conditions are required. For example, a mobile terminal disclosed in JP-A-5-207544 estimates quality of communication (communication quality) based on electric field intensity information, noise information, etc. concerning received radio signals and previously obtained communication quality measurement/judgment results, and displays availability/unavailability of voice communication, availability/unavailability of data communication, maximum data transmission rate (when data communication is available), etc. on its display screen. In a radio communication system disclosed in JP-A-2003-125440, a control station (which determines a radio channel to be used for radio communication between a mobile station and a base station) calculates throughput of each radio channel that can be allocated to a mobile station (whose transmission rate varies depending on the allocated radio channel) and allocates a suitable radio channel to the mobile station based on the calculated throughput.
In such communication systems, a terminal can move from a cell (an area covered by radio waves transmitted by a base station) to another cell while communicating with a base station of each cell, therefore, a technique called “handover” or “handoff”, for maintaining the communication even when the terminal moves across cell borders, becomes necessary. In CDMA radio communication systems, a technique called “soft handover” or “soft handoff” is used, in which a mobile terminal simultaneously communicates with a plurality of base stations and the communication is maintained by selecting a base station in the best communication status from the base stations simultaneously communicating with the mobile terminal (see 3GPP TR25.832 Chapter 5.2 and 3GPP2 A.S0003-A (Version 2.0) Chapter 5.4, for example).